The dynamic range of the human eye is capable of a remarkable ten orders of magnitude—from near darkness to bright sunlight. The sensitivity of the human eye can largely be attributed to the division of photoreceptors in the retina between rod cells, which are capable of responding to a single photon of light, and less sensitive cone cells. Both rod and cone cells are capable of dynamically altering their sensitivity, thus further enhancing the range of sensitivity of the human eye.
The retina of the human eye is also exemplary for the combination of phototransduction and signal processing at the focal plane. The retina is organized in a three-dimensional structure (3D) with separate layers for phototransduction and signal processing. Biological retinas efficiently use limited bandwidth by only encoding salient features and digitally transmitting through a spike-based code. This 3D retinal structure allows for maximization of the area for phototransduction (e.g., fill-factor) while having area left over to perform low level signal processing.
Work is ongoing to develop sensors for use in digital devices such as digital cameras that have the sensitivity range of the human eye. For example, recent advances in semiconductor technology have allowed the manufacture of sensors that approach the dynamic range of the human eye. In particular, avalanche photodiodes (APD), which are the solid state equivalent of photoreceptor cells found in the human eye, have been fabricated using techniques similar to those used to fabricate commercial chips. However, while solid state sensors, such as APDs, which are capable of approximating the dynamic range of the human eye have been developed, there is still a need to develop a circuit for dynamically operating an APD and/or an array of APDs in a manner that mimics the human eye's ability to dynamically adjust operation mode to a wide dynamic range of light conditions.